1. Field of the Invention
This invention relates to a toner composition for use in electrophotography. More particularly, this invention relates to a toner composition for use in electrophotography which, in both indirect and direct developing methods of electrophotography, is uniformly charged with a strong positive electric charge thereby permitting clear development of negative electrostatic latent images without any fogging.
2. Description of the Prior Art
As is known well, xerography is a type of electrophotography which uses static electricity. Likewise there have been practiced several methods for developing the electrostatic latent image which is formed on the surface of an insulating substance. Of these methods, the most widely commercially used methods are the cascade method in which glass beads are used as a carrier for the toner, and a magnetic brush method in which iron powder is used as the carrier. In both of these two methods, the toner particles are charged by friction and are dusted onto the electrostatic latent image by means of electrostatic attraction.
A toner is a colored resin powder which, when charged by friction either by the cascade method or the magnetic brush method, should be charged uniformly either completely negative or completely positive, and it should, when developed form a clear image free from fog or indistinct margins on the visible image. In addition, various other properties are generally required for toners, including the following:
THE ELECTRIFICATION (OR CHARGING CAPABILITY) OF THE TONER SHOULD NOT BE INFLUENCED GREATLY BY HUMIDITY, BUT RATHER IT SHOULD ALWAYS BE SUBSTANTIALLY CONSTANT UNDER VARYING HUMIDITY CONDITIONS TO ENSURE STABLE CHARGING;
THE TONER SHOULD BE MELT SHARPLY AT A RELATIVELY LOW TEMPERATURE (HOWEVER, TONERS WHICH CAUSE CAKING OR ARE VISCOUS AT NORMAL TEMPERATURE (20.degree. TO 25.degree.C.) are not preferred);
the surface of a photosensitive layer should be washed readily;
it should have good abrasion resistance;
it should have good fixability to a sheet of paper to form a copy;
it should have a proper fluidity;
it should admix well with a colorant;
it should not generate an unpleasant smell or poisonous gases during developing; and
the resin per se should readily be pulverized to a particle size of 5 to 20 microns.
When the non-exposed portion of the surface of the photosensitive layer, prior to the developing step, is charged with a negative electric charge after charging and exposure, a positive picture cannot be obtained unless it is developed with a toner which is charged with a positive electric charge. Most resin powders imparted with friction by the cascade method using glass beads as the carrier or by the magnetic brush method using iron powder as the carrier, however, are charged with a negative charge, and very few of them are charged with a positive electric charge.
There has heretofore been known, as a positive-charging toner, a composition wherein a colorant (such as carbon black) and a charge-control agent are blended with a thermoplastic binder resin. If the composition according to the conventional method comprises only a two-component system consisting of the thermoplastic resin such as polystyrene or polyester resin and the colorant, and it does not contain the charge-control agent, the composition can be charged with only a small negative electric charge or a small positive electric charge by friction thereof with the carrier. In order to impart to the toner a strong negative or strong positive electric charge, therefore, it is necessary to add a charge-control agent. In order to impart to the toner a strong negative charge, for example, there is known the method disclosed in Japanese Patent Publication No. 26478/70 in which an azo-type oil-soluble dye containing a complex of chromium is added as a charge-control agent. Alternatively, in order to give a strong positive charge, a basic dye is added.
One of the primary difficulties and disadvantages of these methods for controlling frictional charging by the use of the charge-control additive is that the additive does not admix well in the thermoplastic resin and it does not disperse uniformly therein. As a consequence the surfaces of the resulting toner particles are not charged uniformly, either negative or positive, thereby causing fogging or trailing phenomena at the development and thereby preventing the obtaining of clear and sharp developed images.